Health & Safety and Technical notes

A voltameter improvised from two burettes (as illustrated) is perfectly adequate for this demonstration.

Alternatively, a Hoffman voltameter could be used but the gas pressure may be too low to blow bubbles.

Procedure

a Fill the large gas jar with 0.1 M sulfuric acid almost to the top.

b Hang the electrode assembly over the rim.

c Lower the burettes into the acid with the taps open until the open ends rest on the bottom of the jar. If the acid level does not reach the taps, connect a syringe to the top of the outlet, and transfer the remaining air to the syringe.

d Close the top and lift the burette to surround one electrode.

e Repeat with the other burette and support them in clips as shown.

f Connect the voltameter into a series circuit of rheostat, ammeter and DC supply.

g Switch on. A current will now flow. The rheostat can be adjusted to give a suitable current of about 0.5 A. Bubbles will be seen at both electrodes and gas can be collected in the inverted burettes.

Teaching notes

1 The relatively high solubility of oxygen in water makes it preferable to run the equipment for some time before the demonstration if acceptable volumes of oxygen are to be obtained. If the current is too low, the inverted burettes should be raised a little.

2 With a Y-piece and thin rubber tubing, the two gases can be mixed in a soap bubble. (To help form the bubble, the burette taps are opened and the burettes lowered in the voltameter so that the gases are forced out.) If the bubble is exploded, the energy transfer will be apparent.

3 To test the gases, they can be extracted quite easily in separate syringes as follows. Connect the syringe to the top of the burette, open the tap, and withdraw the plunger of the syringe until the syringe is full of gas. Then fill test tubes with the gas. Do this by putting a test tube full of water upside down in a trough of water and connecting a rubber tube to the syringe. This can be put under the mouth of the test tube. Gas can be propelled into the test tube by gently pushing the plunger inwards. Test the gases in the usual way: for hydrogen using a lighted splint, and for oxygen using a glowing splint.

4 Miniature versions of this demonstration using test tubes instead of burettes will turn it into a class experiment.

5 The appearance of bubbles at both electrodes suggests that there may be ions travelling both ways; the hydrogen carrying positive charge and the oxygen negative charge.